Many electronic devices include one or more rechargeable batteries that require external power to recharge from time to time. Often, these devices may be charged using a similar power cord or connector, for example a universal serial bus (“USB”) connector. However, despite having common connection types, devices often require separate power supplies with different power outputs. These multiple power supplies can be burdensome to use, store, and transport from place to place. As a result, the benefits of device portability may be substantially limited.
Furthermore, charging cords may be unsafe to use in certain circumstances. For example, a driver of a vehicle may become distracted attempting to plug an electronic device into a vehicle charger. In another example, a charging cord may present a tripping hazard if left unattended.
To account for these and other shortcomings of portable electronic devices, some devices include an inductive energy transfer device. The user may simply place the electronic device on an inductive charging surface of a charging device in order to transfer energy from the charging device to the electronic device. The charging device transfers energy to the electronic device through inductively coupling between a transmitter coil in the charging device and a receiver coil in the electronic device. In some situations, an inductive energy transfer device can adversely impact the operations of a touch sensing device in an electronic device that includes both a touch sensing device and an inductive energy transfer device.
FIG. 1 illustrates a simplified block diagram of a prior art transmitter device and a receiver device in an inductive energy transfer system. The charging device 102 (“transmitter device”) includes a transmitter coil 104 that couples inductively with a receiver coil 106 in the electronic device 108 (“receiver device”) to transfer energy from the transmitter device to the receiver device. At certain frequencies, noise produced by the transmitter device 102 can adversely impact a touch sensing device 110 in the receiver device 108 when a user touches an input surface for the touch sensing device 110 while the transmitter device is transferring energy to the receiver device (e.g., to charge the battery 112). The noise can overwhelm the measurements obtained by the touch sensing device and make it difficult to discern a touch measurement from the noise. The noise can reduce or effectively destroy the resolution of the touch sensing device.
For example, in some embodiments the touch sensing device is a capacitive touch sensing device that detects touch through changes in capacitance measurements. When the user touches the input surface of the touch device (e.g., with a finger 114), a parasitic capacitance exists between the finger and an earth ground 116. A parasitic capacitance (represented by capacitor 122) also exists between the AC-to-DC converter 118 and the earth ground 116. Common mode noise produced by the DC-to-AC converter 120 in the transmitter device 102 can couple to the receiver device through the parasitic capacitance CP. The common mode noise produces a noise signal IN that produces a varying voltage across the capacitor 122. The touch by the finger 114 is input with respect to the earth ground 116, but the touch sensing device 110 measures capacitance CSIG with respect to a device ground. Effectively, the varying voltage across the capacitor 122 interferes with the capacitive touch measurement and makes it difficult to discern the touch measurement from the noise.